In the medical, biopharmaceutical, pharmaceutical or paramedical fields, but also in other fields such as in particular that of food, the problem arises of the mixing of large volumes of fluids in clean or sterile conditions.
The mixing of liquids in clean or sterile conditions is a common operation which must often meet the following criteria:                the maintenance at the lowest possible level of an initial biocharge in the constituents of the mixture and in the mixture itself;        as limited as possible an endotoxins content in the constituents of the mixture and in the mixture (and often the combination of the two criteria above);        in more critical conditions, also the maintenance of the sterility of the constituents of the mixture and of the resulting mixture;        in even more special conditions, in addition to the above criteria, the protection of the operators from the potential toxicity of the constituents of the mixture and of the mixture itself.        
Currently, this type of mixing operation is generally carried out using stainless steel tanks fitted with a stirrer immersed in said tanks.
The cost of these tanks is such that they must, naturally, be reusable. This involves a certain number of associated disadvantages and risks.
The most significant of these risks are the following:                the need to prepare the tank in advance, which involves on-site cleaning, disinfection and pre-sterilization (CIP or Clean In Place) and on-site sterilization operations, for example using flowing steam (SIP or Steam In Place);        the use of a tight-seal tank with a mixer, typically a mixer with propellers, immersed in the tank, necessitates the use of a stuffing box around the shaft of the mixer in order to make the system tight. Pre-sterilization and sterilization operations are costly, and these operations must be revalidated regularly, taking into account the ever increasing demands of the regulatory, health or other authorities;        a risk of bacterial contamination, either through the opening of the tank, or through the stuffing box, even due to operator error;        finally, the usual risk of cross-contamination between batches, which is a risk inherent in the re-use of equipment, may be cited.        
It would therefore be desirable to have available a mixing system, preferably simple in design and structure, which satisfies all or some of the criteria mentioned above.
U.S. Pat. No. 5,941,635 describes a system comprising a bag fitted with several dip tubes, in which the liquid to be mixed is sampled in the middle of the bag, using one of the dip tubes, and said liquid returns via the bottom of the bag, by means of an internal tube comprising a mixing head, connecting the top and the bottom of the bag.
Apart from its complexity, this system involves operation inside the bag during its production, in particular in order to insert the dip tubes and the mixing head. There is therefore a significant risk of introducing pyrogenic particles.